Doppler blood flow system and method using low frequency noise signal processing

Abstract

A Doppler blood flow system and method in which an ultrasonic wave is reflected off of red blood cells in blood flowing in tubing. The reflected ultrasonic wave is digitally processed in the frequency domain. The flow rate of the blood is directly related to the average frequency of the Doppler signal. The typical Doppler signal is not a single frequency representing a single flow rate but is really an entire range of frequencies in which certain frequencies predominate which are indicated by increased amplitude of the signal when plotted in the frequency domain. The typical Doppler signal may actually have two "humps", one at a higher frequency which represents the actual information bearing content of the signal related to flow rate and one at a lower frequency which is caused by vibrations in the blood tubing system itself and could be caused by motor noise from the blood pump. Since the flow rate is related to average frequency of the information bearing portion of the typical Doppler signal, the low frequency "hump" must be taken out of the Doppler signal in order to obtain an accurate flow rate.

What is claimed is: 
     
       1. For use in a system having a fluid containing particles flowing at a rate of flow having a value through a tube, a method of determining said value of said rate of flow of said fluid, comprising the steps of: transmitting an ultrasonic signal through said tube at an oblique angle thereto;   receiving the ultrasonic signal which has been reflected off said particles contained in said fluid creating a received ultrasonic signal having time domain characteristics;   converting said received ultrasonic signal into a frequency domain signal having frequency domain characteristics;   analyzing said frequency domain signal for a low frequency hump;   removing said low frequency hump from said frequency domain signal creating a modified frequency domain signal having an average frequency;   determining the average frequency of said modified frequency domain signal; and   calculating said value of said rate of low of said fluid from said average frequency of said modified frequency domain signal using Doppler techniques.   
     
     
       2. For use in a system having a fluid containing particles flowing at a rate of flow having a value through a tube, an apparatus for determining said value of said rate of flow of said fluid, comprising: an ultrasonic signal transmitter arranged to transmit an ultrasonic signal through said tube at an oblique angle thereto;   a receiver arranged to receive the ultrasonic signal which has been reflected off of said particles contained in said fluid having a received ultrasonic signal having time domain characteristics as an output;   converter means coupled to said receiver and receiving said received ultrasonic signal for converting said received ultrasonic signal into a frequency domain signal having frequency domain characteristics;   analyzer means coupled to said converter means and receiving said frequency domain signal for analyzing said frequency domain signal for a low frequency hump;   signal processing means coupled to said analyzer means for removing said low frequency hump from said frequency domain signal creating a modified frequency domain signal having an average frequency;   determining means coupled to said signal processing means for determining said average frequency of said modified frequency domain signal; and   calculating means coupled to said determining means and receiving said modified frequency domain signal for calculating said value of said rate of flow of said fluid from said average frequency of said modified frequency domain signal using Doppler techniques.